NASA has released a technical report on potential future search efforts
for near-Earth objects after a year of analysis by scientists working
on this issue. This Science Definition Team was chartered to study what
should be done to find near-Earth objects less than 1 kilometer in
size. While impacts by these smaller objects would not be expected to cause global devastation, impacts on land and the tsunamis resulting from
ocean impacts could still cause massive regional damage and still pose a
significant long-term hazard.

In 1998 NASA commenced its part of the "Spaceguard" effort, with the
goal of discovering and tracking over 90% of the near-Earth objects
larger than one kilometer by the end of 2008. An Earth impact by one of
these relatively large objects would be expected to have global
consequences and, over time scales of a few million years, they present the greatest impact hazard to Earth. Approximately 60% of the estimated 1,000 to 1,200 large near-Earth objects have already been discovered, about 45% since NASA efforts started, and each of the five NASA-supported search facilities continue to improve their performance, so there has been good progress toward eliminating the risk of any large, undetected impactor.

To understand the next steps to discovering the population of
potentially hazardous asteroids and comets whose orbits can bring them into the Earth's neighborhood, NASA turned to this Science Definition Team of 12 scientists. The Team, chaired by Dr. Grant Stokes of the MIT Lincoln
Laboratory, was asked to study the feasibility of extending the search
effort to the far more numerous, perhaps hundreds of thousands, of
near-Earth objects whose diameters are less than one kilometer.

NASA considers the Science Definition Team's findings to be
preliminary, and a much more in-depth program definition, refining objectives and estimating costs, would need to be conducted prior to any decision to continue Spaceguard projects beyond the current effort to 2008.

The link below will allow a download of the complete Science Definition
Team report (pdf format) and the Executive Summary of this report
follows.

A Study to Determine the Feasibility of Extending the Search for
Near-Earth Objects to Smaller Limiting Diameters

In recent years, there has been an increasing appreciation for the
hazards posed by near-Earth objects (NEOs), those asteroids and periodic
comets (both active and inactive) whose motions can bring them into the
Earth's neighborhood. In August of 2002, NASA chartered a Science
Definition Team to study the feasibility of extending the search for
near-Earth objects to smaller limiting diameters. The formation of the team was motivated by the good progress being made toward achieving the
so-called Spaceguard goal of discovering 90% of all near-Earth objects (NEOs) with diameters greater than 1 km by the end of 2008. This raised the question of what, if anything, should be done with respect to the much
more numerous smaller, but still potentially dangerous, objects. The team
was tasked with providing recommendations to NASA as well as the
answers to the following 7 specific questions:

What are the smallest objects for which the search should be optimized?

Should comets be included in any way in the survey?

What is technically possible?

How would the expanded search be done?

What would it cost?

How long would the search take?

Is there a transition size above which one catalogs all the objects, and below which the design is simply to provide warning?

Team Membership

The Science Definition Team membership was composed of experts in the
fields of asteroid and comet search, including the Principal
Investigators of two major asteroid search efforts, experts in orbital dynamics, NEO population estimation, ground-based and space-based astronomical optical systems and the manager of the NASA NEO Program Office. In addition, the Department of Defense (DoD) community provided members to explore potential synergy with military technology or applications.

Analysis Process

The Team approached the task using a cost/benefit methodology whereby
the following analysis processes were completed:

Population estimation - An estimate of the population of near-Earth
objects (NEOs), including their sizes, albedos and orbit distributions,
was generated using the best methods in the current literature. We
estimate a population of about 1100 near-Earth objects larger than 1 km,
leading to an impact frequency of about one in half a million years. To the
lower limit of an object's atmospheric penetration (between 50 and 100
m diameter), we estimate about half a million NEOs, with an impact
frequency of about one in a thousand years.

Collision hazard - The damage and casualties resulting from a collision
with members of the hazardous population were estimated, including
direct damage from land impact, as well as the amplification of damage
caused by tsunami and global effects. The capture cross-section of the
Earth was then used to estimate a collision rate and thus a yearly average
hazard from NEO collisions as a function of their diameter. We find
that damage from smaller land impacts below the threshold for global
climatic effects is peaked at sizes on the scale of the Tunguska air blast
event of 1908 (50-100 m diameter). For the local damage due to ocean
impacts (and the associated tsunami), the damage reaches a maximum for
impacts from objects at about 200 m in diameter; smaller ones do not reach the surface at cosmic speed and energy.

Search technology - Broad ranges of technology and search systems were
evaluated to determine their effectiveness when used to search large
areas of the sky for hazardous objects. These systems include
ground-based and space-based optical and infrared systems across the currently credible range of optics and detector sizes. Telescope apertures of 1, 2, 4, and 8 meters were considered for ground-based search systems along with space-based telescopes of 0.5, 1, and 2 meter apertures. Various geographic placements of ground-based systems were studied as were space-based telescopes in low-Earth orbit (LEO) and in solar obits at the Lagrange point beyond Earth and at a point that trailed the planet Venus.

Search simulation - A detailed simulation was conducted for each
candidate search system, and for combinations of search systems working
together, to determine the effectiveness of the various approaches in
cataloging members of the hazardous object population. The simulations were accomplished by using a NEO survey simulator derived from a heritage
within the DoD, which takes into account a broad range of "real-world"
effects that affect the productivity of search systems, such as weather,
sky brightness, zodiacal background, etc. Search system cost - The cost
of building and operating the search systems described herein was
estimated by a cost team from SAIC. The cost team employed existing and
accepted NASA models to develop the costs for space-based systems. They developed the ground-based system cost estimates by analogy with existing systems.

Cost/benefit analysis - The cost of constructing and operating
potential survey systems was compared with the benefit of reducing the risk of an unanticipated object collision by generating a catalog of
potentially hazardous objects (PHOs). PHOs, a subset of the near-Earth objects, closely approach Earth's orbit to within 0.05 AU (7.5 million
kilometers). PHO collisions capable of causing damage occur infrequently, but the threat is large enough that, when averaged over time, the anticipated yearly average of impact-produced damage is significant. Thus, while developing a catalog of all the potentially hazardous objects does not
actually eliminate the hazard of impact, it does provide a clear risk
reduction benefit by providing awareness of potential short- and long-term
threats. The nominal yearly average remaining, or residual, risk in
2008 associated with PHO impact is estimated by the Team to be
approximately 300 casualties worldwide, plus the attendant property damage and destruction. About 17% of the risk is attributed to regional damage from smaller land impacts, 53% to water impacts and the ensuing tsunamis, and 30% to the risk of global climatic disruption caused by large impacts, i.e. the risk that is expected to remain after the completion of the
current Spaceguard effort in 2008. For land impacts and all impacts
causing global effects, the consequences are in terms of casualties, whereas for sub-kilometer PHOs causing tsunamis, the "casualties" are a proxy for property damage. According to the cost/benefit assessment done for this report, the benefits associated with eliminating these risks
justify substantial investment in PHO search systems.

PHO Search Goals and Feasibility

The Team evaluated the capability and performance of a large number of
ground-based and space-based sensor systems in the context of the
cost/benefit analysis. Based on this analysis, the Team recommends that the next generation search system be constructed to eliminate 90% of the
risk posed by collisions with sub-kilometer diameter PHOs. Such a system
would also eliminate essentially all of the global risk remaining after
the Spaceguard efforts are complete in 2008. The implementation of this
recommendation will result in a substantial reduction in risk to a
total of less than 30 casualties per year plus attendant property damage
and destruction. A number of search system approaches identified by the
Team could be employed to reach this recommended goal, all of which have
highly favorable cost/benefit characteristics. The final choice of
sensors will depend on factors such as the time allotted to accomplish the
search and the available investment (see Figures 9.3 and 9.4).

Answers to Questions Stated in Team Charter

What are the smallest objects for which the search should be optimized?

The Team recommends that the search system be constructed to produce a catalog that is 90% complete for potentially hazardous objects (PHOs)
larger than 140 meters.

Should comets be included in any way in the survey? The Team's analysis
indicates that the frequency with which long-period comets (of any
size) closely approach the Earth is roughly one-hundredth the frequency
with which asteroids closely approach the Earth and that the fraction of
the total risk represented by comets is approximately 1%. The relatively
small risk fraction, combined with the difficulty of generating a
catalog of comets, leads the Team to the conclusion that, at least for the
next generation of NEO surveys, the limited resources available for
near-Earth object searches would be better spent on finding and cataloging
Earth- threatening near-Earth asteroids and short-period comets. A NEO
search system would naturally provide an advance warning of at least
months for most threatening long-period comets.

What is technically possible? Current technology offers asteroid
detection and cataloging capabilities several orders of magnitude better than the presently operating systems. NEO search performance is generally
not driven by technology, but rather resources. This report outlines a
variety of search system examples, spanning a factor of about 100 in
search discovery rate, all of which are possible using current technology.
Some of these systems, when operated over a period of 7-20 years, would
generate a catalog that is 90% complete for NEOs larger than 140 meters
(see Figure 9-4).

How would the expanded search be done? From a cost/benefit
point-of-view, there are a number of attractive options for executing an expanded search that would vastly reduce the risk posed by potentially hazardous object impacts. The Team identified a series of specific groundbased, space-based and mixed ground- and space-based systems that could accomplish the next generation search. The choice of specific systems will depend on the time allowed for the search and the resources available.

What would it cost? For a search period no longer than 20 years, the
Team identified several systems that would eliminate, at varying rates,
90% of the risk for sub-kilometer NEOs, with costs ranging between $236
million and $397 million. All of these systems have risk reduction
benefits which greatly exceed the costs of system acquisition and
operation.

How long would the search take? A period of 7-20 years is sufficient to
generate a catalog 90% complete to 140-meter diameter, which will
eliminate 90% of the risk for sub-kilometer NEOs. The specific interval
depends on the choice of search technology and the investment allocated.

Is there a transition size above which one catalogs all the objects,
and below which the design is simply to provide warning? The Team
concluded that, given sufficient time and resources, a search system could be constructed to completely catalog hazardous objects with sizes down to
the limit where air blasts would be expected (about 50 meters in
diameter). Below this limit, there is relatively little direct damage caused
by the object. Over the 7-20 year interval (starting in 2008) during
which the next generation search would be undertaken, the Team suggests
that cataloging is the preferred approach down to approximately the
140-meter diameter level and that the search systems would naturally
provide an impact warning of 60-90% for objects as small as those capable of producing significant air blasts.

Science Definition Team Recommendations

The Team makes three specific recommendations to NASA as a result of
the analysis effort:

Recommendation 1 - Future goals related to searching for potential
Earth-impacting objects should be stated explicitly in terms of the
statistical risk eliminated (or characterized) and should be firmly based on
cost/benefit analyses.

This recommendation recognizes that searching for potential Earth
impacting objects is of interest primarily to eliminate the statistical risk
associated with the hazard of impacts. The "average" rate of
destruction due to impacts is large enough to be of great concern; however, the event rate is low. Thus, a search to determine if there are potentially hazardous objects (PHOs) likely to impact the Earth within the next few hundred years is prudent. Such a search should be executed in a way that eliminates the maximum amount of statistical risk per dollar of
investment.

Recommendation 2 - Develop and operate a NEO search program with the
goal of discovering and cataloging the potentially hazardous population
sufficiently well to eliminate 90% of the risk due to sub-kilometer
objects.

The above goal is sufficient to reduce the average casualty rate from
about 300 per year to less than 30 per year. Any such search would find
essentially all of the larger objects remaining undiscovered after
2008, thus eliminating the global risk from these larger objects. Over a
period of 7-20 years, there are a number of system approaches that are
capable of meeting this search metric with quite good cost/benefit
ratios.

Recommendation 3 - Release a NASA Announcement of Opportunity (AO) to
allow system implementers to recommend a specific approach to satisfy
the goal stated in Recommendation 2.

Based upon our analysis, the Team is convinced that there are a number
of credible, current technology/system approaches that can satisfy the
goal stated in Recommendation 2. The various approaches will have
different characteristics with respect to the expense and time required to
meet the goal. The Team relied on engineering judgment and system
simulations to assess the expected capabilities of the various systems and
approaches considered. While the Team considers the analysis results to
be well-grounded by current operational experience, and thus, a
reasonable estimate of expected performance, the Team did not conduct analysis at the detailed system design level for any of the systems considered.

The next natural step in the process of considering a follow-on to the
current Spaceguard program would be to issue a NASA Announcement of
Opportunity (AO) as a vehicle for collecting search system estimates of
cost, schedule and the most effective approaches for satisfying the
recommended goal. The AO should be specific with respect to NASA's position on the trade between cost and time to completion of the goal.